ISTANBUL - Turkey's military fired an artillery round into Syria on Sunday in immediate retaliation after a shell fired from Syria landed in the Turkish border town of Akcakale, broadcasters said, the second such incident in five days.

Prime Minister Tayyip Erdogan warned Syria on Friday that Turkey would not shy away from war if provoked, but a series of mortar bombs fired from Syria have hit Turkey since then.

There were no casualties when the latest Syrian shell hit land near a plant belonging to the Turkish Grain Board (TMO), several hundred meters from the center of Akcakale, where five civilians were killed on Wednesday in previous Syrian shelling.

The exchanges are the most serious cross-border violence in Syria's conflict, which began as pro-democracy protests, but has evolved into a civil war with sectarian overtones.

NATO member Turkey was once an ally of Syrian President Bashar Assad but turned against him after his violent response to an uprising in which, according to the United Nations, more than 30,000 people have died.

The Syrian Observatory for Human Rights said the shells fired from Turkey landed near the Syrian town of Tel Abyad.

Broadcaster NTV said the shell from Syria landed in the garden of the TMO plant near storage silos. It said the silos had suffered some damage from shrapnel.

Akcakale had been quiet since Turkey's retaliation on Wednesday and Thursday for the initial shelling, but Syrian government forces began shelling areas around the Syrian town of Tel Abyad on Sunday morning, Dogan news agency reported.

Before the latest strike into Turkey, the Syrian military had fired seven artillery shells on Sunday into an area close to the Syrian customs building, which is around 300 meters from the border and under the control of rebel forces, Dogan said.

People were reportedly killed in those strikes and two Syrians wounded in the strikes were carried through the border fencing and taken to a hospital in Akcakale, Dogan said.